1. Field of the Invention
At least one embodiment of the invention relates to a method for producing an anode electrode for an aluminum electrolytic capacitor, an anode electrode obtainable by the method, and also an aluminum electrolytic capacitor containing such an anode electrode.
2. Description of the Related Art
An aluminum electrolytic capacitor is a capacitor of which the anode electrode (also referred to as the anode foil) consists of aluminum, on which a uniform, electrically insulating aluminum oxide layer in the form of a dielectric is produced by anodic oxidation. Generally, a liquid or solid electrolyte forms the cathode of the capacitor. Typically, anhydrous electrolytes are used almost exclusively, of which the conductivity is much lower than water, such that an internal resistance of the capacitor increases. Conventional aluminum electrolytic capacitors therefore generally, include a second aluminum foil as a current feed to the electrolyte, wherein the aluminum foil is typically electrically separated from the anode electrode by a separator permeable to the electrolyte. Generally, the second aluminum foil is also referred to (actually incorrectly) as a cathode foil.
The anode electrode consists of a foil made of highly pure aluminum. Generally, the foil is roughened in an electrochemical process in order to increase the effective anode surface. Due to this roughening process, the effective surface may be increased up to approximately 150 times compared to a smooth surface.
After the roughening process, the aluminum foil is anodically oxidized. Thus, an electrically insulating oxide layer (Al2O3) is formed on the aluminum surface by applying a current source in correct polarity in an electrolytic bath, wherein the insulating oxide layer constitutes the dielectric of the capacitor.
The roughened and anodically oxidized anode foil and the roughened cathode foil are cut to a desired width during a further course of the manufacturing process, generally from a parent roll, and are then cut to length. The foils are then connected to aluminum strips placed transversely relative to the foils in order to contact terminals of the capacitor. Together with two paper strips as a separator and as a reservoir for the electrolyte, the foils are then usually wound into a coil. The coil of the capacitor with the terminals led out is then saturated under vacuum with the electrolyte in a subsequent production step. This fully matches the structure of the anode foil and of the dielectric located thereon and thus makes the surface enlargement of the anode capacitively effective for the first time. The saturated, or impregnated coil is installed in an aluminum casing, provided with a sealing plate, and is fixedly mechanically closed by flanging. The capacitor is then freed from defective points in the dielectric (healed up) by reforming.
Aluminum electrolytic capacitors correspond to plate capacitors, of which the capacitance, inter alia, is greater, the greater is the electrode surface. The total capacitance of the aluminum electrolytic capacitor is determined definitively by the magnitude of the anode capacitance, since the capacitance of the cathode foil is generally much smaller. High capacitances and power densities may therefore be achieved by enlarging the effective surface of the anode foil.
As described above, the enlargement of the effective surface is achieved in commercial products by etching a porous surface structure. Here, a compact aluminum foil of constant thickness is used as a starting point, and the structure is formed subsequently. Surface factors of up to 150 are achieved. However, a further increase of the effective surface, even by optimization of the etching processes, is typically no longer possible. Heavier etching generally leads to a loss of mechanical integrity of the material. The use of a thicker starting material also does not lead to an enlargement of the effective surface, since areas close to the surface are typically lost as a result of the etching process.
There is thus a considerable need to further increase the capacitance of aluminum electrolytic capacitors. In particular, it would be desirable if the effective surface of the anode electric could be increased further.